Traveling wave tube



5 Sheets-Sheet 1 f-z ""w R. w. PETER TRAVELING WAVE TUBE 0N .Pf/W77, .DfMF/TY Feb. 16, 1960 Filed Jan. 4, 1954 R. W. PETER TRAVELING WAVE TUBE Feb. 16, 1960 3 Sheets-Sheet 2 VFiled Jan. 4, 1954 ....lll l I IN VEN TOR.

ATTORNEY Feb. 16, 1960 R. w. PETER 2,925,515

TRAVELINGWAVE TUBE Filed Jan. 4, 1954 3 Sheets-Sheet 3 INI/ENTOR.

ATTORNEY 2,925,515 TRAVELING WAVE TUBE Rolf W. Peter, Cranbury, NJ., assignor to Radio Corpo` The present invention relates to improvements in electron tubes and circuits, and particularly to amplier tubes of the traveling wave type.

In a conventional traveling wave tube an elongated wave-guiding or Wave-propagating means, in theform of a section of transmission line with suitable input and output terminals or couplings, is mounted Within an evacuated envelope. The terminals are sealed through the envelope or otherwise adapted to be coupled to external transmission lines. The wave-guiding means is designed as a delay line along which electromagnetic waves are guided at a traction of the Velocity of light. The form of delay line most commonly used in ltraveling wave tubes is a conducting helix, such as a helical metal coil of uni- -form diameter and pitch. An electron beam is projected by suitablemeans along, and usually coaxial with, the

helix at a beam velocity substantially equal to or slightly higher than the axial wave Velocity along the helix. Suitable means is provided for preventing undue spreading of the beam due to space charge eiects. In operation of the tube as an amplifier a signal wave traveling along the helix creates electromagnetic elds therealong which interact with the-electrons in the beam to produce electron velocity modulation and consequent electron bunching. As the wave and beam travel synchronously along the helix the bunched beam induces electric iields and currents along the helix, causing the amplitude of the `wave to increase exponentially. The electron beam gives up direct current energy 4to the helix, thus producing an amplified signal at the output end of the tube.

In any beam type amplifier tube the signal-to-noise ratio at the output end of the tube is` dependent in` part upon the amount of noise initially present in the beam.Y

In traveling wave tubes this initial noise is amplified along with the signal by the interaction between the beam and the helix or other wave-guiding means.

The principal object of the present invention is to provide improved means for reducing the overall noise factor of traveling wave ampliiier tubes. y

Most of the methods of decreasing the noise factor of such tubes that have been used heretofore attempt to eliminate or reduce the amount of noise in the beam Y priorto modulation thereof by the signal.

In accordance with the present invention, means are provided in a traveling wave tube tor preventing, or at least substantially reducing, the amplification of the noise tiuctuationspresent in the beam, at least during a first part ofthe length of the Wave-guiding means where the beam is being modulated by the signal. Farther along the wave-guiding means,l after the beam has been modulated to a relatively high level as compared to the noise iluctuations in the beam, amplification of the noise can be tolerated without substantial reduction in signal-tonoise ratio, or increase in overall noise factor, of the tube.

It is known that the noise fluctuations of an electron beam under space-chargelimited conditions leaving the potential originate essentially as velocity uctu- United States Parent ige When such a beam is `used in a conventionalftraveling wave tube the interaction between the beam noise and the wave-guiding means is greatest only in short periodically-recurring regions therealong where. the current density liuct-uations of the beam noise are at maximum amplitude. However, the Vsignal wave traveling along the wave-guiding means interacts with the beam continuously therealong. According to the present invention, the coupling between the beam and the Wave-guiding means in a traveling wave tube is varied or changed periodically along the length thereof in such manner that the coupling is high in regions of minimum noise current uctuations (where a minimum of amplification of noise will occur)` and either low or zero in regions of maximum noise current fluctuations, at the center frequency of the desired `operating bandwidth of the tube. As a result, the beam is modulated by the signal wave in the regions of high coupling with a minimum amplification of the noise of signal frequency in the beam.

The `coupling between the beam and the Wave-guiding means can be varied or changed, in accordance with the invention, in several ways. The distance or spacing be- Patented` Feb. 16,- 1960 tween the beam and the wave-guiding` means can be` located along thebeam path at noise current minima and shielding the beamfrom` the modulating fields in other regions. The coupling mayV also be varied by using as the Wave-guiding ,means a pair o-f coupled delay lines,`

such as coaxial helices. The wave energy fed into one helix is transferred periodically between the helices as the wave travels therealong, andthe coupling between the beam and the lwave varies therealong with the radiot frequency potential `on the nearest helix.

When it is desired to vary the coupling by varying the beam diameter, a hollow wave-guiding means, such asf a wire helix or a baille-loaded,` waveguide is preferred. r

The variation in beam diameter may be produced by focusing the beam by `means of either an axial magnetic lield of suitable intensity or `a suitable periodical electrostatic lens structure.` When the dimensions of the wave` guiding means are to be varied, the latter may be a wire helix, with or without supplementary coupling means, a

disc-onrod structure with a hollow surrounding beam,

or a baille-loaded wave-guide, for examples.

Actually, an electron beam has a random velocity dist -tribution at the cathode which resultsin random noise velocity modulation of the electrons in the beam over a very broad range of frequencies. However, it is only necessary to 4avoid appreciable amplification of noisetin the frequency range for whichwthe tube is designed to opcrate. The points of maxima and minima of the noise or plasma standing wave can be calculated from known Y formulas, or located by probing along a beam in an ex-` perimental tube with .a resonant cavity tuned to the center frequency ot the desired bandwidth. traveling wave tube with variable coupling it is preferable to locate the input end ofthe helix, or other wave-guiding means, at or near the first noise current minimum from the cathode. As pointed out above, the coupling may be varied `along the entire length of vthe tubebut is prefer` ably varied only .for a distance suicientto produce sub-p. stantial modulation of the beam by the signal to establish I a sufliciently high signal-to-noise ratio.

In designing a i plifier tube be made lower at the input end than at the output end, to reduce ampliiication of noise during the initial modulation ofthe beam.. However, noattempt hasbeen made. heretofore to vary the coupling periodically,'with.the coupling low in regions of high noise currentand `high in regions of low noise current.

VIn the drawing:

Fig 1 is a schematic view of the standing wave pattern along an electron beam;

Fig.'2 isi a graph showing schematically how the cou- 'pling between the beam and the wave-guiding means in a traveling V wave tube may be varied along the beam path in' accordance with the invention;

Fig. 3 isa longitudinal sectional view of a traveling wave tube embodying the present invention;

Fig. 4 is a similar View Vof'another embodiment of theinvention;

Fig. S'isa similar View of still another embodiment;

Fig. .6..is' a transverse section taken on the line 6-6 of Fig. 5;

Fig. 7 is a fragmentary view of a modification of Fig. Y5; f

Fig". 8 is atransverse sectional view taken on the line 8-8 of .Fig 7;

Fig. 9 isa longitudinal sectional view of a traveling wave tubeof the plural-cavity type embodying the invention, .and

Fig.Y 10 is a fragmentary view of a further modification of Fig. 5.

- Referring now to the drawings in detail, Fig. l follows Hahn and vRamo (S. Ramo, Phys. Rev., vol. 56, p. 276, 1939) andillustrates a stationary space-charge standing wave pattern produced by an alternating perturbation onr amoving electron beam of uniform diameter with highzand-low-alternating current density regions as indicated. The transverse planes` at which the alternating currentdensity maxima and minima occur are indicated bythe lettersa, b,.c, d, e, etc. lThe distance between successive maxima, or successive minima, is half of a.V

plasma wavelength, kp, that is, 'the wavelength of the space-charge standing wave pattern. Fig. 1 is general, and appliesto a standing pattern produced either by velocity modulation of the beam by a signal of given frequency -or'by noise velocity fluctuations of a given frequency `originating lat the cathode. beam noise, the noise velocity fluctuation, or alternating current.` velocity, is `a maximum and the noise-current fluctuation, or. alternating current density, is a minimum, at the origin: of. the Vvelocity fluctuation, that is, at or near the, cathode surface. Y function 'of beam velocity,ramong other variables, and hence doesgnotbecome uniform alongfthe beam path until afterthe beam' has been accelerated by the gun electrodes to the operating velocity. Hence, the point a 1n YFlg. 1 represents some low density region at or beyond the final accelerating electrode of the gun.

Fig. 2 shows schematically how `the coupling or interaction between the beam and the helix, for example, in a traveling wave tube may be periodically varied, along at least a part of the length of Vthe helix, in accord ance with theinvention, to provide high coupling in regions of -low noise current, for modulation of the beam bythe Vsignal on the helix, and low Vcoupling in regions Vof. high noise current to avoid amplification of the noise initially present in the beam. The upper curve representsthe noise current standing wave, corresponding to the solidwcurveinpFig. l, but on a different scale, and the lower, curve represent the variation in coupling. 'ille wavelength, Ac, of Vthe'couplihg wave is approximately equal to Ap/Z. vIlle phase angle `between .the periodicities of Amand )tp should be kept as Vlow as possible. The `coupling lvariation could be continued throughout the length of the'helix,but ,this would `result in an un- In the'case ofV The plasma wavelength is a necessary loss in gain or amplification of signal. v There'- iore, it ispreferableto terminate-the..couplingreduce tions as soon as the signal-to-noise ratio in the beam is sufciently high that amplication of the noise can be tolerated without increase in the overall noise factor of the tube.

Fig. 3 shows the invention embodied in a helix-type traveling wave tube. The tube comprises essentially an elongated dielectric envelopeA 1 containing a helix 3 of uniform diameter and pitch, an electron gun 5 adapted to project an electron beam'B through the helix 3,'and external electromagnets 7, 8 vand 9, or other suitable means, for establishing an axial focusingmagnetic field of suitable intensity along the beam path. The gun 5 is designed to produce a beam that is initially divergent at the cathode in which case the axial magnetic field deflects the electrons moving in paths at an angle thereto and causes the beam to be periodically focused and defocused. Thus, the beam envelope is scalloped, with successive maximum and minimum diameters, as shown in Fig. 3. The first electromagnet 7 is designed to produce a strong magnetic field along the gun 5 and a predetermined part of the helix 3. Due to the variation' in beam voltage within the gun 5 the distance between maximum diameter regions varies up to the helix. Within the helix this distance is uniform for a given magnetic ield. This focusing and defocusing of the beam causes the distance or spacing between the beam and the surrounding helix 3 to be periodically varied along the helix,

resulting in variable coupling therealong. In accordance with the invention, the strength of the magneticy field of electromagnet 7 is chosen relative to the other parameters to make the focus or coupling wavelength, Ac, equal to half the noise or plasma wavelength, Ap, of the'beamV for the center frequency of 4the amplification bandwidth of the tube, and to cause the minimum ldiameter regions A of the beam to coincide with the noise current maxima l along the beam. Moreover,- the input end of the helix is preferably located `substantially at a noise currentV minimum, or coupling maximum. The electromagnet 7 extends along the beam for at least two noise current minima, or beam diameter maxima, as shown in Fig. l3. Beyond electromagnet 7 is a short transition electromagnet 8 adapted to produce a magnetic field somewhatv weaker than that of electromagnet 7. From electromagnet 8 to the end of the beam path is an electromagnet 9 adapted to produce a relatively weak magnetic field.

n 4In the transition electromagnet 8V the focusing force is The divergent beam gun structure 5 is essentiallyV the same as thelow-noise gun disclosed and claimed in my copending -application Serial No. 385,064, tiled October 9, 1953, now Patent No. 2,909,704, granted October 20, 1959, assigned to the same assignee as the present l application, and comprises a cathode 11, a concave focus` ing ring 13 having its inner edge located behind theremis-Y sive surface' of the cathode, a first accelerating electrode 15, and one or morel other'accelerating electrodes 17.

The last accelerating electrode 17 is connected *for directV currents to the helix- 3Vby a coil 19 and a coupling sleeve 21. By using the low-noise gun shown in Fig. 3,'the

beam noise is reduced Yin additionto the advantages of the periodical couplingin preventing Vamplilication of theA beam'noise. However, the periodical coupling'can -be used'with other gun structures.

The coupling sleevezl and asimilar coupling sleeve 23 at the output end'of the'helix 3 are connected to the `helix by helix extensions 3f and 3" which are coupled respectivelyY to external input and output waveguides 25"` and 27. Bahfthe. electromagnets 7 and 9 are. Inail@4 The variation in beam diameter `can be applied to any tube having a hollow wave-guiding means through which the beam is projected. Fig. 4 shows the invention appliedto a baie-loaded waveguide type which is suitable for use with electro-static focusing, instead of magnetic focusing. In this tube the delay line comprises a series of centrally-apertured metal discs 31 connected together for radio frequency currents at their outer edges by metal rings 33. The discs and rings are arranged in groups, of four discs in each ring as shown, for example, separated by electrical insulation discs 35, to permit thef application of a different direct-current potential to each group for electrostatic focusing purposes. As indicated on the drawing, alternate groups are maintained at a given potential, V0, and the other valternate groups are maintained at a different potential, VniAV. Any conventional electron gun 5 which will project a suitable beam along the axis into the waveguide may be used.

'I'he small and large diameter regions of the beam are' correlated with the noise current maxima and minima, respectively, as in Fig. 3. Preferably, the beam focusing is produced by only about two groups of discs, beyond which the metal discs 31 are maintained at the same direct-current potential. Suitable means, such as a shielded electromagnet may be provided to prevent the beam from spreading beyond the two groups. Instead of an electro-magnet, the remaining metal discs 31 could be separated into insulated groups of smaller axial length to provide electrostatic focusing with shorter focal length and-smaller variations in beam diameter.

`I n Figure 4 the rings 33 are sealed to the insulation discs l35 and serve Las part of the vacuum envelope of the tube. portion 37 sealed to the rst disc 31 by a Kovar ring 38, and a collector 39 sealed to the last disc 31 bya dielectric ring 41 and intermediate Kovar rings 43 and 45. The input and output couplings of the tube may be waveguides 47 and 49 coupled to the input and output ends of the tube by dielectric Windows 51 and 53.

In the embodiment shown in Figure 5 an electron `gun 5-projectsa substantially uniform diameter electron beam along the axis of a helix 55 of variable diameter, Whereby the spacing between the beam and the helix is varied, along at least a part of the length of the helix. The ends of the helix 55 are coupled to external input and output waveguides or resonators to that shown in Figure 33. The helix 55 is preferably constructed with a small diameter at the input end and periodically changing diameter for two maximum diameter portions as shown. Each of the minimum portions are located substantially at a noise current minimum of thenoise standing wavelength along thebeam for the center frequency of the operating bandwidth of the tube, to obtain maximum coupling `at those portions. Beyond the second maximum diameter portion the diameter is kept constant at a relatively small diameter throughout the remaining portion of the helix, `for high coupling to the beam. .Y

The helix 55 may be supported by means of three longitudinally extending insulating strips 57, of ceramic, for example, which are scalloped to fit the outer envelope of the helix 55. The three strips 57 may be maintained circumferentially in position by ceramic rings 59 having notches for the strips 57, as shown clearly in Figure 6. The gun 5 vmay be any conventional gun which will provide a parallel-how beam havinga diameter somewhat less than the minimum diameter of the helix 55. The gun shown in Figure 3 may be used if the focusing and accelerating electrodes are` maintained at potentials in producing parallel-now rather than divergent'flow. An

' velectromagnet may be providedalong the length of the The envelope -is completed by a dielectric gun in a manner similar beam path for establishing an magnetic eld thee;"f along tokeep the beam `from spreading due to space charge eiects.

Figure 7 and 8 show a modification of Figure 5 in which use is made of the helix 61 of uniform diameter and pitch provided with coupling rings 63 each attached to one turn of the helix by means of a tab 64 and having its central aperture coaxial with the beam path. This type of coupling between the beam and the helix is disclosed and claimed in arcopending application of W. .T Dodds, Serial No. 305,797, filed August 22, 1952,

now Patent No. 2,802,135, granted August 6, 1957, assigned to the same assignee as the present application. In accordance with the present invention, the central apertures 65 in the coupling rings 63 are varied in diameter periodically along at least a predetermined portion of the helix 61, in a manner similar to the variation in diameter of the helix itself in Figure 5, to vary the coupling between the beam and the helix. The apertures in the coupling rings located at or near noise current maxima would have a large diameter and the aperture in coupling rings located near noise current minima would have a small diameter. The helix 61 and associated coupling rings 63 would be substituted in the tube shown in Figure 5 in place of the variable-diameter helix and its supporting ceramic members, the remainder of the tube being the same as that of Figure 5.

In the embodiment of the invention shown in Figure 9 the coupling between the beam and the modulating electric fields is discontinuous, in that a beam is projected by a gun 5 through a series of cavity resonators 67 to a collector 69. Each of the resonators 67 is coupled by a coaxial line 71 to a delay line'73 along which a traveling wave is adapted to be propagated, whereby modulating -electric fields are set up along the beam path across `gaps 75Min the resonators 67. 1 Each of the spaces between the resonators 67 is shielded by a tubular shield 77` surrounding the beam path and connected to the resonators 67. Each of the gaps 75 of the resonators 67 is located substantially ata noise current minimum ofthe noise standing wave inthe beam for the center frequency ofthe operating range of the tube. Thus the noise current maxima` in the beam occur within the shields 77 and midway between the electric iield gaps 75. Therefore, the beam is-coupled to the signal line 73 substantially only in a region of noise current,

thereby avoiding the amplification of noise during modulation of the beam. The .resonators67 and shields 77 serve as a part of the vacuum envelope in a manner similar to the tube of Figure 4. The gun 5 and collector 69 are associated with the other structure as in Figure 4. One or more electromagnets may be provided to prevent the beam from spreading.

The helix structure shown in Figure l0 may be used in place of that shown in Figure 5. 'Ibis structure comprises an inner `helix 7S of uniform diameter and pitch, positioned closely adjacent the inner surface of the dielectric envelope 1, and an outer helix 79 closely surrounding the envelope 1 and inductively coupled through the dielectric `envelope to the inner helix 78. The outer helix 79 has constant diameter and pitch overa predetermined distance A such that the axial wave velocity along the outer helix 79 is substantially the same Ias that along the inner helix 78. Beyond the portion A the outer `helix 79 is gradually increased in pitch toward the output end of the tube. the outer helix 79 may be terminated a short distance beyond the portion A. The input signal is coupled to the inner helix 78 by means of a coupling ring 81 and helix extension 78' Which is adapted to Ibe coupled to an exterior input waveguide in a manner similar to that shown in Figure 3, except that in Figure 10 the sleeve is located beyond the end of the helix. Y u

In operation of the tube of Figure 10, an input traveling wave is introduced to the inner helix by means If desired,

www

results ina periodically-variable coupling between thel beam land the traveling wave. The two helices are designed. so that,- lalong the: portion A, the periodicity or wavelength, .Maof the transfer of .wave energy from one helix tot,the other, ,or/.in other words, the wavelength of the radio frequency standingvwave alonglthe inner helix 478, isequal toene-,half the noisev or plasma wavelength Vof `the noise standingwave in the beam for the center frequency of-the desired operating frequency range of. the tube. The value of M for a given pair of coupled lines can be determined experimentally by introducing awave at the end of one linefand probing along r either; line with a` standing wave detector to determine the pointsot maximum potential, The value of at can also be computed by using the formulas set forth by L. Pipesinva paper entitled The Matrix Theory of Four-Terminal Networks, APhil. Mag.r(3()), 7th Series, p; 370, 1940. Furthermore, since the energy is fed initially Vto Y.the inner helixy 78,- afpotential or coupling maximum occursatV the input end thereof, and hence, the inputend should be locatedsubstantially at a noise currentY minimum orfthenoise standing Wave. The portion A includes at v least two, noise currentA minima. Beyond the portion A,I the lincrease .in the pitch of the outer A.helix-579 reduces the coupling betweenl the two helicesk and causes the radio -frequency potential and, coupling along the inner helix 78 to remainhigh throughout the yremainderofthe length of the tube. Ittwill be understoodjt-hat the energyA may be fed initially tothe outerhelix 797` instead ofthe inner helix 78, in which case the-input end-of the `helix 79 would be ylocated substant-ially ata noisey current maximum. This arrangementV haszthe advantage ithat the input and output transt mission lines can be easily connected directly to the outerhelixf-V v What is claimed is:V l Y t1. A traveling wave tube adapted to operate over a predetermined frequency range comprising an elongated r wave-guiding structure adapted to propagate a traveling wave therealong ata-given `axial velocity which is a fraction of the velocity of light, signal input means directly connected to one end of said structure, signal output.means directly. connected to the other end of said structure1, andmeans for producing along said structure an electron` beam having for any given frequency a noise standing wave with noise'current maxima and minima alternately `located'therealong and having a beam velocity substantiallyequalto or slightly higher than said axial Wave velocity for interaction -withl said traveling'wave, the coupling between 'said structure and said beam being substantiallyfhigherin regions' of noise current minima than in regions of lnoise current maxima of the noise standingwave for the center frequency of said operating range, at least for the iirst two noise current minima and theintermediate noise current maximum of said wave along said structure, whereby during operation said'beam-can bemodulated by a signal wave traveling along saidfstructure inthe,regions of higher coupling -with a `minimum of, amplication of said noise standing wave.-k

2. A traveling wavetube as in claim l, wherein the y ditlerencesgin;,coupling result .,frQm; said structure y and "Said-ahem; beine. Spacedfafther; apart-*iin the regionY of.

Y when awtravellinggwavegis introduced-'into cnenvof two closely coupled helices having substantially/...the same f,

current- 'maximaq i `A3 A traveling wave. tube as inclaimfZ, whereinfthe-a transverse dimensions of said structureare substantially-f A constant,j andVv the' spacing` between Isaid structure and said' beam is varied by varying the ltransversedimensions-- of said beam along saidfstructure.

4. A traveling wave tubeas in claim k2, wherein the@A transverse dimensions -of said beam-are "substantially-f constant, and the -spacing between said structurefand said beam yis varied by'varying the `transverse dimensions ot' said structure `5. A traveling `Wavetubeas inrclaim 1, wherein saidwave-gmiding structure comprises means` for subjecting` said beam -toaxialfradio frequency/modulating electricVv eldsv only in relatively short ,discrete regions located along the beam path at or near said noise current minima,

and means` for shielding saidbeam in other regions,

6. A traveling wavetube as in claim 1, wherein said wave-.guiding structure comprises a series of cavity reso` nators each coupled Vto said beam at or.v near a noise. current minimum, conductive shields surrounding `the.; beam pathl between said resonators, and a delay linecoufr pled to said cavity resonators..-

7. A traveling wave tube as in claim 1,I wherein said;- wave-guiding` structure lcomprises a firstdelay line-ex-l tending along andlcoupled to said beamv and aisecond delay line closely coupled to said iirst delay line and having an axial wavevelocity substantially equal to that of said rst delay line for atleast a half plasma wave.-

length.

8. A traveling wave-tubeas in claim'7, wherein said rst delay lneis a-rst wire.helix,. having agivenf circumference-to-pitch.ratio,4 surrounding said beam, and;

said seconddelay line is a second-Wire helix, Yhaving substantially the Vsame circumference-torpitch ratio at the input end thereof, surroundingsaid irst helix. j

9. A traveling wave tube as inclaim 8,V wherein the. pitch of said secondhelixincreases gradually from said'.

nected toone end of said helix, signal output means directly connected tothe other end of said helix, and

means for projecting a beam of electrons along said helix Y' and having a beam velocity substantially equal to' orslightly higher than .said axial wave Velocity for 'inter` ction withsaid traveling -wave, the coupling between Y said helix and said beam Vbeing periodically variedgalonga predetermined portion of the length of said helix, withl at least two maximum coupling regions and-,at least one intermediate minimium coupling region nearV the inputVv of saidV helix, the rst of said maximum coupling regions `being locatedy substantially atrthe input end of said helix. l2. .A traveling Wave 4tube as in claim 11, wherein saidv coupling vvariation results from said vhelix and said*` beambeing spaced substantially farther apartin said .minimum coupling region Vthan in said maximum coupling regions.

13. A traveling wave tube as in claim l1, wherein-theV transverse dirnensionsof .said helix are substantially constant and lthe ltransverse dimensions ofsaid beam .are Y varied along said portion. Y i4. A traveling wave tube as in transverse dimensions of said beam-are substantially con.

stant and-thektransverse.dimensions `ofsaid helix are.

varied along saidportion.-

l5. A traveling wave tube as in claim 11,` wherein thev transverse. dimensions offsaid beamrand said helix-are., substantially constant, and. thev vcoupling- .between ..rsraid: beam'anthsad hliniS--Varssl brmWHSQffCQuPIQeIPsa.

claim 11, wherein the Y assauts 9 of variable diameter periodically distributed along said helix and surrounding said beam.

l16. A traveling wave tube as in claim 11, wherein said beam is projecting along the central axis of said helix, and said coupling variation is produced by a second helix, surrounding and coupled to said first-named helix, and having an axial wave velocity along said portion substantially equal to that of said first-named helix.

17. A traveling wave tube as in claim 16, wherein the pitch of said second helix is such that the wave velocity therealong increases gradually from said portion.

18. A traveling wave tube as in claim 3, wherein said structure is an elongated helix of constant diameter and pitch and wherein the transverse dimensions of said beam are varied by means for producing an axial magnetic 15 2,828,439

10 field along said helix and means for causing electrons of Ysaid electron beam Yto enter said magnetic field at an angle thereto.

19. A traveling Wave tube as in claim 1, wherein the first of said rst twoV noise current minima occurs substantially at lthe input end of said wave-guiding structure.

20. A traveling wave tube as in claim 1l, wherein the tirst of said maximum coupling regions is located substantially at the input end of said helix.

References Cited in the le of this patent UNITED STATES PATENTS Landauer Feb. 5, 1952 Fletcher Mar. 25, 1958 

